This application claims the priority of 8-211193, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an electronic device, and more particularly to an electronic device structure with excellent corrosion resistivity suitable for an electronic device with lead wires used under the heavily corrosive environment, such as a temperature sensitive resistor for measuring the temperature of intake air in an automobile.
Referring to FIG. 1, a conventional temperature sensitive resistor, i.e., a thermistor, typical of an electronic device with lead wires of the type to which the present invention is addressed will be described below. In particular, a glass-sealed thermistor of an axial type (a diode type) is used as an example. The known thermistor is constructed by putting thermistor element 2 in glass tube 1, which is hermetically sealed by sealing electrodes 3A, 3B. The thermistor element 2 is sandwiched by the electrodes 3A, 3B to maintain the electric contact therebetween. Further, lead wires 4A, 4B are electrically attached to the electrodes 3A, 3B, respectively.
Usually, a dumet wire (JIS H4541) has been used for the sealing electrodes 3A, 3B in such a glass-sealed type thermistor as described above. FIG. 2 shows a sectional view of an example of the dumet wire.
A dumet wire is constructed by coating core wire 11 made of iron-nickel alloy by copper 12 as an intermediate layer, which is further covered by surface layer 13 of cuprous oxide (Cu2O) or borate (Cu2Oxe2x80x94Na2B4O7). The core 11 of a dumet wire is made of iron-nickel alloy in order to bring the thermal expansion coefficient closer to that of glass, whereas the surface layer thereof is made of cuprous oxide for the purpose of the good melting-adhesiveness with glass. Since the sealing electrode 3A, 3B is made by cutting such a dumet wire in an appropriate length, iron-nickel alloy as core material is exposed to the atmosphere at the end surface 3a. 3b. 
Further, the lead wire 4A, 4B, as an example thereof being shown in FIG. 3, is formed by coating the surface of core wire 15 made of dumet, iron or iron-nickel alloy with copper 14.
Metallic portions of the glass-sealed type thermistor, i.e., the outer end surfaces 3a, 3b of the sealing electrodes 3A, 3B and the surfaces of the lead wires 4A, 4B, are plated by solder so as to solder the thermistor onto a substrate. Further, nickeling can also be used to attach the thermistor to the substrate by spot welding or the like.
As described above, the core of the dumet wire made of iron-nickel alloy is exposed to the atmosphere at the end surfaces 3a, 3b. However, the corrosion resistivity of the end surface 3a, 3b can be improved by solder-plating or nickeling.
A thermistor of this kind is often used in a corrosive environment, such as a temperature sensitive resistor for measuring the temperature of intake air in an automobile. In such a case, the thermistor is required to have sufficiently high corrosion resistivity.
However, a conventional glass-sealed type thermistor as mentioned above does not have sufficiently high corrosion resistivity and hence has a disadvantage that corrosion occurs when it is used in a heavily corrosive environment, such as sulfur dioxide gas atmosphere.
In other words, a thermistor, which has solder plating to cover its metallic portions, can not have sufficiently high corrosion resistivity. Compared with a soldered thermistor, a thermistor which has nickeling for the same purpose can be much improved in corrosion resistivity.
Even with the later thermistor, however, corrosion resistivity is less than desirable. This is because when a lead wire is cut for the length adjustment thereof, its core of easily corrosible iron-nickel alloy or iron appears at the cut surface which is exposed to the corrosive atmosphere and from which corrosion will begin.
Further, in the case of using the spot-welding, a layer of nickel plated on a lead wire is melted by welding heat, and the core of iron-nickel alloy or iron is exposed to the corrosive atmosphere and continues to be corroded from such an exposed portion.
An object of the present invention is to provide a structure of an electronic device, such as a thermistor, having extremely high corrosion resistivity.
A structure of an electronic device according to the present invention comprises an element with a desired electronic characteristic, electrodes electrically connected to the element, inorganic insulator for sealing or coating at least part of the element and the electrodes, and lead wires provided for the electrical connection with the electrodes, wherein the lead wires are made of corrosion resistant material and further at least a portion surrounding a joint of the lead wires and the electrodes is coated with corrosion resistant material.
Since in the present invention, the lead wires themselves are made of corrosion resistant material, there occurs no corrosion in a welding portion and a cut portion. Further, since the lead wires and the exposed portion of the electrodes are coated with corrosion resistant material, a structure of an electronic device having the extremely high corrosion resistivity, and hence high durability as well as high reliability, is provided. As a result, an electronic device with a structure according to the present invention can be used for a long period without corrosion in a heavily corrosive environment, such as sulfur dioxide gas atmosphere.
In one of the embodiments of the present invention, an axial type of a glass-sealed thermistor may be constructed in the following manner; namely, first of all, cylindrical electrodes made of dumet wire are welded with lead wires made of nickel. Then, a semiconductor thermistor element and the cylindrical electrodes are put in a glass tube in such a manner that the electrodes hermetically seal both ends of the glass tube. Further, nickeling is performed on an exposed portion of the electrodes and the lead wires, as well as the welded portion of the lead wires with the electrodes.
In another embodiment of the present invention, a linear type of a temperature sensitive resistor with lead wires may be constructed as follows; namely, at first, a temperature sensitive element is made by forming a metallic film on the surface of a cylindrical alumina bobbin. Cap electrodes made of iron-nickel alloy are fitted by pressure to both ends of the temperature sensitive element as formed above. Then, lead wires made of nickel are welded to the cap electrodes. Part of the temperature sensitive element and the electrodes are coated with glass, and nickeling is performed on an exposed portion of the cap electrodes and the lead wires, as well as the welded portion of the lead wires with the cap electrodes.